Nitrogen Dynamics at a Manured Grass Field Overlying the Sumas-Blaine Aquifer in

Whatcom County

• Barb Carey, LHg, Washington State Department of Ecology • Joe Harrison, Washington State University, Professor Livestock Nutrient

Management Program

Ground Water Protection Council Annual Forum Seattle, Washington

October 6, 2014

Background & Study Location •Transboundary, unconfined glacial outwash aquifer

•High intensity agriculture (dairy and berries)

•Shallow depth to water (mostly < 10 feet); 50 feet thick

•Only feasible water source for ~20,000 people on U.S. side (29% > 10 mg/L Nitrate-N)

•Study site: 22-acre grass field

Purpose • Track N through manure/soil/crop/groundwater system over time • Evaluate effectiveness of land application guidance in dairy nutrient

management plans (DNMPs) in protecting GW quality

• Track nitrogen inputs (manure, fertilizer, irrigation water), outputs (grass crop) and residual (soil, groundwater)

• Evaluate 2 possible indicators of groundwater nitrate impacts o N mass balance o Post-harvest soil nitrate

Objectives

Marine Climate = Wet winters: Precipitation mainly in fall and winter: 2.5 - 3.2 feet (2004-2008)

Relatively mild winter temperatures—rarely stay below freezing

Nitrogen Mass Balance

• Inputs • Outputs • Manure • Fertilizer • Irrigation water • Atmospheric input • Soil organic matter

• Grass crop • Volatilization • Denitrification

• Residual

• Soil nitrate • Groundwater nitrate

Dairyman tilled and replanted grass field in April 2004 --just before start of study (NOT part of study plan)

Groundwater Monitoring

•6 shallow wells (12 feet deep)

•Silt loam soil, w/substantial clay in some drilling samples •Sampled monthly, 4.5 years

•Nitrogen (NH4-N, NO2+NO3-N, total N), chloride, organic carbon and related constituents.

Average nitrate-N concentration and N loading in 6 monitoring wells

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Nitrate MCL

2004 2005 2006 2007 2008 2009

Monthly mean groundwater nitrate-N

Totalnitrogen applied annually (lb/acre)

No Data

Total N applied annuallyTillage/ Replant

Results • GW nitrate concentrations highest 1st winter (max 45

mg/L-N) after high manure N loading • GW nitrate decreased with lower manure N loading—

below the MCL (2006-2007)

• Spike in GW nitrate late 2006 after late-season application, and again in 2009 after annual manure loading 2 times the mass removed

• Crop removal consistent despite large variation in N application

1) N Mass balance

(Inputs) - (Outputs) = Excess N to GW 2) Intensive post-harvest soil nitrate testing

(amount of nitrate in soil at the end of growing season)

2 methods for estimating manure nitrogen loading effects on GW

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(mg/L)

MCL/GW Standard

Estimated excessnitrogen

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-79 -78

146

No loadingdata

Groundwater Nitrate-N(Nov-Dec mean of6 shallow wells)

1) Estimated annual excess N vs. early winter groundwater nitrate-N

Estimated excess nitrogen NExcess (bars) calculated using N mass balance vs. annual mean early winter (November-December) groundwater nitrate-N concentrations in 6 monitoring wells (line).

2) Soil nitrate results (1 foot depth) • WSU guidance: Post-

harvest soil nitrate 15 ppm and below (1 foot depth), no management changes needed (30 ppm recommended by WA Dept of Agriculture)

• Soil dynamic system (high variability, high recharge fall/winter), post-harvest soil nitrate tests inherently underestimate the amount leaching to GW (Kowalenko, 1987; Kuipers et al, 2014).

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Soil Nitrate

Fall Target(15 mg/kg or47 lb/acre)

2004 2005 2006 2007 2008

September-Octoberfall sampling period

Soil nitrate (m

g/kg)

Fall Target(15 mg/kg or47 lb/acre)

2004 2005 2006 2007 2008

September-Octoberfall sampling period

Soil nitrate (m

g/kg)

Each measurement = mean of duplicate samples

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Mean w

inter groundwater nitrate-N

(mg/L)

Winter GW nitrate-Nmean in 6 shallowwells

MCL/GW Standard

Maximum of weeklyfall soil nitrateMean of

weekly fall soil nitrate

2) Fall soil nitrate (1 foot depth) vs. groundwater nitrate-N (early winter)

Simple spreadsheet model to estimate mass of nitrate leaching

•Model back-calculates amount of NO3-N needed to produce GW concentrations observed •We know the final concentration and volume and incoming concentration and volume, as well as site conditions, and volume of recharge

Groundwater inflow entering

from upgradient, with a given flow rate and nitrate-N concentration

Groundwater outflow leaving the site, with a given flow rate and

nitrate-N concentration

Recharge at the site over the fall/winter with unknown nitrate-N concentration

Q,CGW Inflow

CLeachate NO3

C GW Outflow NO3 b

dL

W

dH

Recharge (R, CR → NO3R )

Model 1 NO3-LEACHATE

Model 2 GWNO3-FORECAST

1-2 feet (DDF)

dH dL

= iH

Soil sample horizon (0-1 ft) (ρb; CSoil NO3 0-1)

KH ; AFSZ

Vadose zone (AFVZ)

Total residual soluble nitrate-N (NO3TotResidual)

Model 1 Method A

(NO3TotResidual = NO3Residual 0-2 + NO3R + NO3Supp)

Model 1 Method B (NO3TotResidual = NO3MB Residual + NO3Supp)

Saturated mixing zone

NO3Residual 0-2

Groundwater flow direction

Recharge from precipitation and/or irrigation

LInfiltration ,QLeachate

Not to scale

Pitz, 2014--https://fortress.wa.gov/ecy/publications/publications/1403018.pdf

Backcast N loading estimate to GW

• Average 115 lb/acre nitrate leached to GW during the fall/winter period (Range: 42-230 lb/acre) – Average of 33 mg/L nitrate-N leaching

• 40% of leaching occurred after the fall flush

(January – March)—probably freshly mineralized nitrate from soil organic matter

Comparison of #1 and #2 with model results

• Method 1 estimate of soil organic matter contribution large unknown—not predictive

• Soil nitrate very dynamic all year (including winter), only sampled top foot—not predictive

• Backcast model integrates leaching over the season and spatially—based on field measurements

Take home messages • Field N mass balance not correlated with GW

nitrate (soil organic N contribution = wild card)

• Intensive soil nitrate data (1-foot) not correlated with GW nitrate (high variability, fast leaching in winter)

• Backcast model using GW nitrate provided useful estimate of N mass leaching

• Applying manure too late in the season resulted in groundwater nitrate increases

• Nitrate continued to leach in the late winter/early spring

• Groundwater monitoring was the only reliable way to assess nitrate impacts

Acknowledgements Lynden, Washington dairy producer/landowner Whatcom Conservation District Washington Department of Agriculture Washington State University, Puyallup, staff Washington Department of Ecology staff

Information: Barb Carey, Washington State Department of Ecology bcar461@ecy.wa.gov

Presentation based on: Carey, B.M. and J.H. Harrison, 2014. Nitrogen dynamics at a manured grass field overlying the Sumas-Blaine Aquifer in Whatcom County. Washington State Department of Ecology, Olympia, WA. Publication No. 14-03-001. https://fortress.wa.gov/ecy/publications/SummaryPages/1403001.html